Kaolinite is a common mineral, the chief constituent of kaolin or clay. It is a member of the clay mineral group of phyllosilicates which group also includes talc (or soapstone) and pyrophyllite. Clay minerals similar to kaolinite include dickite, nacrite, anauxite, halloysite, montmorillonite (dominant clay in bentonite), beidellite, nontronite, hectorite, saponite, and illite. Clay minerals similar to talc include minnesotaite. All of these minerals are referred to herein as "kaolinite and kindred minerals".
[References (indicated by bracketed numbers) are listed at the end of description.]
Kaolinite has a wide variety of applications in industry, particularly as a paper filler and a coating pigment (5). It is used as an extender in aqueous based paints and ink, a functional additive in polymers and is the major component in ceramics (5). Kaolinite is an inexpensive additive which can improve the properties of the material in which it is dispersed, provided it is able to form stable dispersions. In order to achieve this the surface is usually modified in some way so as to enhance colloid stability in a system whether it be organic, as for polymers, or in aqueous dispersions.
The surface of kaolinite, like other clay minerals, is relatively inert, unlike silica and many other minerals. Kaolinite has two different basal cleavage faces. One face consists of a siloxane surface with very inert --Si--O--Si-- links. The other basal surface consists of an octahedral, gibbsite [Al(OH).sub.3 ] sheet. Both of these surfaces are theoretically electrically neutral. At the edges of a platelet and at the surface ridges the lattice is disrupted and broken bonds occur which are accommodated as OH groups. These edges have been estimated to occupy approximately 10% of the whole kaolinite surface (6).
The hydroxyl groups at the plate edges are considered to be the major reactive sites of clay surfaces including kaolinite (1). Rigorous conditions are required to modify the surface of clay minerals including the use of anhydrous solvents, fluidised beds or autoclaves, the use of coupling agents, encapsulation by polymer synthesised on the surface or adsorption of surfactants from aqueous suspensions (7, 8, 9). Some of these methods are time consuming, polluting and of high cost and often the adhesion between the kaolinite particles and the modifying chemical is not strong as they are not covalently bound over the whole surface but either electrostatically bound or encapsulated (i.e. sheathed) and only bound covalently at the edges of the kaolinite particles. This can mean that the modifying chemical on the surface of kaolinite may not remain attached under certain conditions, (eg. during processing of the polymer into which it has been dispersed) causing the dispersion to become unstable. Methods, such as acid hydrolysis, have been attempted in the past to increase the reactivity of clays, including kaolinite, but these have been unsuccessful on kaolinite (1).